For the LORD had said to Moses, “Tell the Israelites, ‘You are a stiff-necked people. If I should go with you for a single moment, I would destroy you. Now take off your jewelry, and I will decide what to do with you.’”
The LORD repeats that Israel is stiff-necked and says even a moment of His presence could destroy them, so they must remove their jewelry while He decides what to do.
This verse explains why their mourning takes visible form.
1Then the LORD said to Moses, “Leave this place, you and the people you brought up out of the land of Egypt, and go to the land that I promised to Abraham, Isaac, and Jacob when I said, ‘I will give it to your descendants.’ 2And I will send an angel before you, and I will drive out the Canaanites, Amorites, Hittites, Perizzites, Hivites, and Jebusites. 3Go up to a land flowing with milk and honey. But I will not go with you, because you are a stiff-necked people; otherwise, I might destroy you on the way.” 4When the people heard this bad news, they went into mourning, and no one put on any of his jewelry. 5For the LORD had said to Moses, “Tell the Israelites, ‘You are a stiff-necked people. If I should go with you for a single moment, I would destroy you. Now take off your jewelry, and I will decide what to do with you.’” 6So the Israelites stripped themselves of their jewelry from Mount Horeb onward.